1. Field of the Invention
The present invention relates to a modem.
2. Description of the Prior Art
The prior art delta modulation (DM) process converts a continuous signal into an alternating binary signal. This enables mass audio information to be stored in the form of compressed data by using sampled audio data techniques. In delta modulation, an audio signal under consideration at a first sample time is encoded as a logic "1" if the amplitude of the signal is less than that of the audio signal at the next sample time and as a logic "0" if the amplitude is higher.
The principle of DM encoding is exemplified by the prior art DM encoder 30 of FIG. 1 which includes a band pass filter 1; an adder 2; a digitizer 3; a one-clock delay circuit 4; and an integrator 5. The DM encoder 30 makes the comparison between an input value and a predicted value and then outputs a binary pulse having a DM value of either V or--V when the input signal changes to or from a logic "0". This single-bit encoding does not provide an allocation of a code for a continuous constant amplitude input signal.
The encoded signal is demodulated by a DM decoder 32 shown in FIG. 2 which includes an adder 6, a one-clock delay circuit 7, and a band pass filter 8. For a continuous constant amplitude input signal, it is unavoidable that the DM decoder 32 outputs a binary signal of the continuous one/zero alternating pattern showen in FIG. 3. This type of signal is an audio distortion called granular noise because it has an alternating digital pattern even though the input audio signal does not change. This granular noise is usually eliminated by an external low pass filter or a band pass filter.
An alternate method of reducing the noise is to reduce the sampling rate. For instance, dropping the sampling rate from 16 KBPS (kilobits per second) to 8 KBPS, reduces the frequency of the granular noise from 8 kHz to 4 kHz or 1/2 the sampling rate. However, the filter requirements still demand filters having steep response characteristics.